1. Field of Invention
This invention relates to radar systems having digital moving target indicators, and more particularly to prefiltering of the return radar analog video signal prior to presentation to the MTI digital filters.
2. Description of the Prior Art
The use of moving target indicator (MTI) radars to detect moving targets within a scanned sector by detecting the change in amplitude, or phase of the Doppler signal frequency of such targets, is well known in the art. In such systems the reflected radar energy is received from targets in the sector and processed to provide an analog video signal having component Doppler frequencies dependent upon the nature of the target. These signal components are described generally as consisting of small amplitude, high frequency signals from moving targets, and large amplitude, low frequency signals from relatively stationary targets. The MTI radar systems are capable of discriminating between small, varying amplitude signals (denoting moving targets) and relatively fixed amplitude return signals indicative of stationary targets, and generally referred to as clutter. The clutter component signal represents the major portion of the return radar analog video signal, such that moving target signals are masked by a clutter component signal which may be in the order of 20 to 60 db greater. The effectiveness of the MTI system depends upon its ability to filter out the unwanted clutter portion of a return signal so that moving target signals of the lowest order may be detected.
A number of clutter filtering techniques are known in the art. These include delay line cancellers which delay the return video signals of each main bang pulse by one pulse repetition period (PRP) and cancel the fixed amplitude components of a subsequent PRP return to provide moving target indication. Other systems, commonly known as range-gated moving target indicators (RG-MTI), quantize the return signal portion of the PRP into range intervals or bins, providing Doppler, or band pass filters within each range bin to filter out the lower frequency clutter component and pass only the higher frequency moving target components. In the past, the delay line cancellers, or range-gated band pass filters have been of the analog type consisting of passive impedences (such as capacitors, resistors and inductors, whether alone or distributed), and active networks including amplifiers. However, such devices usually have broad, rather than sharp, roll off characteristics, and inherently are extremely difficult and costly to make adaptable. Some of the more recent MTI radar systems employ digital filters, which are implemented with digital signal handling techniques and the use of Z transform equations. Such digital filters can produce very sharp roll off characteristics and are easily modified to vary the attenuation response, however, they are expensive and complex. One of the single most expensive components in a digital filter MTI system is that of the analog-to-digital (A/D) converter which converts the return radar analog video signal into a digital signal equivalent for presentation to the digital MTI filter and detection circuitry (generally of the range-gated band pass type). Due to the extremely low moving target-to-clutter signal ratios (typically minus 20 to minus 60 db) the dynamic range, or total bit capacity of the A/D converters must be high to insure a converter resolution accuracy which is capable of providing amplitude discrimination between the moving target signal component and the clutter component, to preserve the target signal component in the digital word. These A/D converters, which typically comprise twelve bits, or more, are one of the single most expensive components in the system. Therefore, the cost and the ability to provide a sufficiently high dynamic range A/D converter, present the chief problem and performance limitation of the present state of the art MTI systems employing digital filters.